For all their advantages, diesel-fuelled engines have a significant disadvantage. When burned substantially in a diffusion combustion mode, diesel fuel generates high levels of some pollutants. Pollutants such as oxides of nitrogen (NOx) and particulate matter (PM) can be problematic. Cleaner burning gaseous fuels such as natural gas, hydrogen, ethane, propane, blends of gaseous fuels such as blends of natural gas and hydrogen, as well as others tend to expel fewer pollutants than diesel fuel when burned in an internal combustion engine. It has been determined that some gaseous fuels can also provide similar power output when directly injected at high pressure upon completion of a compression stroke or near the commencement of a power stroke in, for example, a four-stroke engine.
While providing emissions benefits, gaseous fuels tend to need some type of ignition assist to initiate combustion when used in an engine with diesel engine compression ratios. One common ignition assist is a pilot fuel provided to a charge to create an auto-ignitable charge that can be used to help initiate combustion of the gaseous fuel when the gaseous fuel is directly injected, as noted above.
Ultimately, however, while providing emissions benefits, combustion of gaseous fuels do generate NOx. In particular, diffusion combustion resulting from gaseous-fuelled direct injection engines tends to generate higher NOx levels than is the case with combustion of a premixed gaseous charge. However, techniques are available for lowering NOx in such an engine. It has been found that exhaust gas recirculation (EGR) can be an important method for reducing NOx in a compression ignition direct injection engine.
Ideally, ever increasing concentrations of EGR in the intake charge would be used in gaseous-fuelled direct injection engines. That is, more EGR tends to reduce NOx. However, as EGR concentrations increase, emissions penalties in relation to other emissions occur. In particular, hydrocarbons (HC), carbon monoxide (CO) and particulate matter (PM) all start to increase with ever increasing levels of EGR. Therefore, EGR for compression ignition direct injection engines have tended to be limited to 30% before other emission become prohibitive.
The present method is applicable to gaseous fuelled compression ignition direct injection engines (or direct injection engines that use a main fuel requiring a auto-ignitable pilot fuel to assist in ignition of the main fuel) for reducing overall emissions by using high EGR levels (beyond 30% by mass concentration in the intake charge) while managing other emissions that frequently arise with high levels of EGR.